Preparation of metal chalcogenides

ABSTRACT

A method embodiment involves preparing single metal or mixed transition metal chalcogenide using exfoliation of two or more different bulk transition metal dichalcogenides in a manner to form an intermediate hetero-layered transition metal chalcogenide structure, which can be treated to provide a single-phase transition metal chalcogenide.

RELATED APPLICATIONS

This application claims benefit and priority of U.S. provisionalapplications Ser. No. 62/605,102 filed Aug. 1, 2017, and Ser. No.62/710,190 filed Feb. 12, 2018, the entire disclosures and drawings ofwhich are incorporated herein by reference.

CONTRACTUAL ORIGIN OF THE INVENTION

This invention was made with government support under Grant No.DE-AC02-07CH11358 awarded by the U.S. Department of Energy. Thegovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to the preparation of a transition metalchalcogenide material that includes using one or more dry orliquid-assisted mechanical exfoliation steps.

BACKGROUND OF THE INVENTION

Bulk single-phase transition metal dichalcogenides (TMDCs), which arebuilt from separate transition metal-chalcogen layers bound together byweak Van der Waals forces, find numerous applications, as components oflubricating oil additives, self-lubricating coating materials,photoelectrode materials, photocatalysts, battery materials, componentsof supercapacitors, thermoelectric materials, and hydrogen storagematerials, catalysts for the electrochemical hydrogen evolution reaction(HER), and more. In the vast majority of cases known to date, TMDC basedmaterials consist of two chemical elements, a transition metal and achalcogen, which form a binary chemical compound.

Recent interest in tunable materials for quantum electronics, energygeneration and storage created a strong demand for multi-principalelement TMDC systems, which performance can be tuned by altering thematerials chemical composition. Currently, the set of known single-phaselayered TMDCs is limited to two and a few examples of three or fourelement systems with general formula of MXX′ and (M,M′)XX′, where M andM′ is selected from the following group of elements Mo, W, Nb, Ti, Zr,Hf, Ta, Re, Pd, Pt, In, Ga or Sn, and X and X′ is selected from S, Se,Te. Mixed (multi-principle element) TMDCs, such as (Mo,W)S₂, (Mo,W)Se₂,(Mo,W)SSe and others find numerous applications as lubricating oiladditives [2], self-lubricating coating materials [3,4], photoelectrodes[5], photocatalysts [6-8], battery materials [9], supercapacitorelectrodes [10,11], thermoelectric materials [12], hydrogen storagematerials [13,14], catalysts for the electrochemical hydrogen evolutionreaction [8]. In particular, two-dimensional (2D) nanostructures ofTMDCs, have attracted considerable interest due to their uniquemechanical, electrical and optical properties. Remarkably, thesematerials provide all favorable mechanical properties of graphene, butin contrast to its near zero bandgap exhibit a direct bandgap in themonolayer form, which can be tuned in mixed TMDCs such asMo_(x)W_((1-x))S_(y)Se_((1-y)), where x and y=0-1, for example.

Conventionally, single-phase TMDCs are prepared using severalexperimental approaches. The reactions of transition metals ortransition metal oxides with pure chalcogens S, Se or Te in bulk orusing chemical vapor deposition technique (CVD) by far dominate thesynthetic landscape.

The preparation of mixed TMDCs is usually carried out using severalexperimental approaches. Chemical Vapor Deposition and Atomic LayerDeposition (CVD & ALD) are by far the major techniques to obtainmonolayers of various types. CVD&ALD utilize the gas phase reactions ofmetals [15] or metal oxides [16-18] with volatile chalcogens in an inertgas atmosphere at elevated temperatures. However, the materials formedusing vapor deposition techniques are often complex systems wheredifferent TMDC constituents are present as separate phases. Anothersynthetic approach to the synthesis of mixed TMDCs involves thedecomposition of co-crystallized thiosalts, such as (NH₄)₂MoS₄,(NH₄)₂WS₄ [19] or other similar precursors [21], in a reducingatmosphere. Thus prepared bulk materials were reported to consist oftwo-cation sulfide layers Mo_(x)W_((1-x))S₂ if the precursor salts aremixed in the solution and treated with electrical current [22] orhydrogen at high temperature [19], or multi-layer single cationheterstructures [20] when a step by step deposition and thermaldecomposition of specific thiosalts is employed. However, recent resultsshowed that the presence of a nano-carbon/graphene support is criticalfor the formation of such solid solutions [23].

An earlier attempt to make single-phase mixed TMDCs by heating upcrushed and thoroughly mixed bulk WS₂ and MoS₂ was reported asunsuccessful [19]. Even at 1000° C. the mechanical mixture of thesemetal disulfides did not form a single-phase solid solution and remainedjust a mixture of the separate WS₂ and MoS₂ phases.

Mechanical processing in the form of milling or grinding is commonlyused for crushing, mixing or exfoliating of TMDCs or precursors fortheir synthesis as well as for making their nanoparticles.

-   -   In one such example, MoS₂ nanosheets are produced in a jet mill        followed by processing with H₂O₂ and ethanol, then upper layer        is filtered and vacuum dried to obtain the so-called 2H—MoS₂        concentrate [24].    -   The high temperature self-lubricating composite based on Ni with        5% C and 40% WS₂ additives is prepared first by ball milling to        obtain uniform mixing and then subsequent annealing of the        latter under elevated temperatures and pressures [25]. Similar        material can be obtained based on copper [26] as well.    -   Mechanochemical exfoliation to produce 2D materials from        graphite, TMDCs or other layered materials is based on        intercalation of Li⁺ and Et₄N+ ions within the layered crystals        in deep eutectic solvent environment by means of shear forces        during ball milling [27].    -   Mechanical activation of WO₃ and sulfur was used to produce        homogeneous hexagonal WS₂ nanoplates upon subsequent annealing        [28,29].

Also, there are reports describing the preparation of TMDC-containingcomposites by employing mechanical milling (mechanical processing) as anexperimental technique:

-   -   Ball milling of a mixture containing WS₂ or MoS₂ and graphite        produces a composite consisting of intermixed TMDC and graphite        particles. Upon milling the uniform distribution of single-phase        WS₂ and MoS₂ nanoparticles ranging in size from 10 to 60 nm and        coated in graphite is obtained [30].    -   Coupling of ball milling technique with sonication in deionized        water environment facilitates production of nano WS₂ and MoS₂        granules [31].    -   The composite of MoS₂—C which is made by ball milling of the        bulk MoS₂ with graphite consists of separate nanographene and        MoS₂ phases, whereby separate graphene layers cover MoS₂        nanoparticles [32].    -   Transition metal chalcogenides are prepared by high-temperature        synthesis with subsequent ball milling in presence of a        surfactant to obtain ultra-thin single-phase nanosheets for the        use as lubricating oil additives [2].    -   Nanocomposite of titanium dioxide and tungsten sulfide        (TiO₂—WS₂), generated by ball milling, shows a high ability to        absorb visible light and, as a consequence, suitability for        water-splitting applications [5].    -   Pretreatment by ball milling is used to generate WS₂ nanopetals        which subsequently act as a base for growing curly MoS₂        structures with a significant photocatalytic activity [6].    -   Ball milling with subsequent heat-treatment at elevated        temperatures is utilized to prepare WS₂ nanosheets-carbon        composite which demonstrated high reversible capacity when        applied as anode for sodium and lithium ion batteries [9].    -   Ball milling of MoS₂ is used for the exfoliation of MoS₂ for        further use as electrode materials for flexible supercapacitors        [10]. In another case, similar electrodes are fabricated from        nanoparticles of Fe₃O₄ attached to nano-WS₂, which are made by        the exfoliation of bulk WS₂ using ball milling and sonication        [11].    -   Ball milling with subsequent sonication and sintering is used        for the preparation of TiS₂—MoS₂ composites that can be utilized        as low-cost high-efficiency thermoelectric materials [12].    -   A ball milled MoS₂ is used in an Mg/MoS₂ nano-composite for        hydrogen storage applications. The nano-composite shows the        reduced desorption temperature and an ability to maintain        nano-scale crystallites, which favors its cycling        stability[13,14].    -   Influence of milling media and materials of milling setup on        exfoliation of MoS₂ has been studied. Ball milling using alumina        balls is found to be more efficient than processing in        all-stainless steel setup [33].    -   Exfoliation of MoS₂ has been achieved by dry milling of the bulk        material with NaCl and subsequent ultrasonic disintegration of        the nanostructured MoS₂ in an organic solvent [34].

Another particular synthetic process that is known uses chemicalconversion of co-crystallized thiosalts, such as (NH₄)₂MoS₄, (NH₄)₂WS₄or other similar precursors in a reducing atmosphere at temperatures of300-700° C. (see references 19, 21, 35-37). One such process includesthe dissolution of (NH₄)₂MoS₄ and (NH₄)₂WS₄ in water, mixing togetherthe aqueous solutions formed, the evaporation of water and subsequentdecomposition of the obtained solid material in hydrogen containingatmosphere at about 400° C. As a result, (NH₄)₂MoS₄ and (NH₄)₂WS₄ arechemically converted into mixed materials as shown in Eq. 1.

(1-x)(NH₄)₂MoS₄ +x(NH₄)₂WS₄+H₂=2NH₃+(Mo_(1-x)W_(x))S₂+2H₂S   (1)

There is a need for a simple and inexpensive method to prepare metalchalcogenides that are single phase. Furthermore, single-phase metalchalcogenides combining four or more chemical elements in theirstructure do not appear to have been reported in the open literature.

SUMMARY OF THE INVENTION

The present invention provides method embodiments for preparing atransition metal chalcogenide material by exfoliating two or moredifferent bulk transition metal chalcogenides separately or together andcombining in a passive manner (self-assembly) and/or active manner(mixing) the exfoliated products to form a transition metal chalcogenidehetero-structure having layers of different composition.

A particular method embodiment of the present invention involvespreparing the metal chalcogenide material starting from different, bulktransition metal dichalcogenides (TMDCs) wherein the method includes oneor more dry or liquid-assisted mechanical exfoliation steps. The bulkTMDCs can include two or more bulk metal sulfides, selenides,tellurides, or their combinations. Embodiments of the present inventionenvision producing a mixed metal or a same (single) metal chalcogenidematerial.

An illustrative method embodiment of the invention involves one or moreexfoliation steps that involve dry mechanical exfoliation by mechanicalprocessing of the two or more different bulk TMDC materials togetherunder inert gas atmosphere for a time period effective to form amaterial, which can have a disordered structure or ordered structure.The ordered TMDC can be a substantially homogenous three dimensional(3D) hetero-layered structures of the two or more unique constituents.

In another illustrative method embodiment of the invention, the drymechanical exfoliating step is conducted by mechanical processing usingpestle and mortar, shaker ball mills of any configuration, planetaryball mills of any configurations, any type of laboratory or industrialgrinders, or other milling, or grinding equipment that can produceplastic, shear and other irreversible deformation as well as partial orcomplete exfoliation of the bulk TMDCs to produce an intermediate 3Dhetero-layered structure. The hetero-layered structure so produced canbe subjected to reactive interlayer mixing and conversion of the layeredstructure into a single phase, wherein reactive intermixing is effectedby means of heat treatment (annealing), or mechanicalworking/processing, or high uniaxial external pressure, or a combinationof external pressure and shear stress, or by cold, or hot rolling, or bycombination of thereof.

Another illustrative method embodiment of the invention involves one ormore liquid-assisted exfoliation steps that involve exfoliating two ormore different transition metal chalcogenide materials together orseparately in the same or in a different liquid medium using mechanicalprocessing, sonication, or any other exfoliation technique and combiningthe products of the exfoliating in a manner to form a transition metalchalcogenide material having hetero-layers (3D heterostructures) ofdifferent composition. The material can be further processed to obtainan ordered multi-principal element single-phase product in a crystallineform.

In a particular illustrative embodiment of the invention, theexfoliating step is conducted by mechanical processing and/or sonicationin the same or different liquid(s) to form dispersion(s), and thecombining step involves spontaneous self-assembly (e.g. in the samedispersion or by mixing the dispersions together), or additive assembly(e.g. by layer-by-layer deposition, spin coating, ink jet printing etc.of the dispersions) of the exfoliated products of the dispersions. Thehierarchical layered structures so produced can be subjected to reactiveinterlayer mixing and conversion of the hierarchical layered 3Dheterogeneous-structures (hetero-structures) into a single phasematerial, wherein reactive intermixing is effected by means of heattreatment (annealing), mechanical working/processing, high uniaxialexternal pressure, or a combination of external pressure and shearstress, or by cold or hot rolling or by combination of thereof.

The present invention also provides a composition of matter embodimentcomprising two dimensional (2D) single layer or three dimensional (3D)multi-layer thick, mixed TMDC nanoplates having a chemical compositionrepresented by (M_(a)M² _(b)M³ _(c . . . n))(X_(d)X² _(e)X³ _(f)), wherethe formula unit includes more than two different metals (M), and X, X²and X³ represent S, Se, and/or Te, whereby the sum of a+b+c+ . . . n isbetween 1 and 3 and the sum of d+e+f is between 1 and 6. For acomposition including more than two different metals (M_(a)M_(b)), thecomposition can be represented by M_(a)M² _(b)X_(d) where X is at leastone of S, Se, and Te.

The present invention further envisions in still another composition ofmatter embodiment a mixed TMDC material, such as a layered bulk materialor nano-structured material such as 2D or 3D nano-plates), having achemical composition represented by (M_(a)M² _(b)M³_(c . . . n))(X_(d)X² _(e)X³ _(f) . . . X^(m) _(z)), where the formulaunit includes four or more different chemical elements; where M is atransition metal selected from the group consisting of Ti, Zr, Hf, Nb,Ta, Mo, W, Re, Pd, Pt, In, Ga and Sn and where X is selected from thegroup consisting of S, Se, and Te and may additionally and optionallyinclude O, N, B, C and/or P; where the sum of a+b+c+ . . . n is between1 to 10; and where the sum of d+e+f+ . . . z exceeds 0.1.

The present invention also provides still another composition of matterembodiment a mixed TMDC material, such as a layered bulk ornano-structured material such as 2D or 3D nano-plates, having a chemicalcomposition represented by (M_(a)M² _(b)M³ _(c . . . n))(X_(d)X² _(e)X³_(f) . . . X^(m) _(z)), where the formula unit includes three or moredifferent chemical elements; where M is a transition metal selected fromthe group consisting of Ti, Zr, Hf, Nb, Ta, Mo, W, Re, Pd, Pt, In, Gaand Sn and where X is selected from the group consisting of S, Se, andTe with Te always present in the material and may additionally andoptionally include O, N, B, C and/or P; where the sum of a+b+c+ . . . nis between 1 to 10; and where the sum of d+e+f+ . . . z exceeds 0.1.

Practice of the present invention utilizes solid-state processing toobtain hetero-layered materials in different TMDC systems, whichmaterials can be converted to single-phase material, and enables simple,inexpensive, and scalable way of their industrial production.

Other advantages of the present invention will become readily apparentto those skilled in this art from the following detailed description,wherein only the preferred embodiment of the invention is shown anddescribed, simply by way of illustration of the best mode contemplatedof carrying out the invention. As will be realized, the invention iscapable of other and different embodiments, and its several details arecapable of modifications in various obvious respects, all withoutdeparting from the invention. Accordingly, the drawings and descriptionare to be regarded as illustrative in nature, and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a and 1b show powder X-ray diffraction patterns for thestoichiometric 1:1 (molar) mixture of MoS₂ and WS₂ milled for variousperiods of time in a planetary mill (FIG. 1a ) and a mixer mill (FIG. 1b).

FIGS. 2a-2d are SEM secondary (SEI) and backscattering (BEI) images ofthe 1:1 (molar) MoS₂ and WS₂ samples milled for 15 hours in theplanetary mill (FIGS. 2a, 2c ) and the mixer mill (FIGS. 2b, 2d ).

FIG. 3 shows EDS elemental mapping of the 1:1 (molar) MoS₂ and WS₂sample milled in the planetary mill for 15 hours where view “a” showsthe analyzed area, view “b” shows sulfur distribution, view “c” showstungsten distribution, and view “d” shows molybdenum distribution.

FIGS. 4a and 4b are TEM images of the 1:1 (molar) MoS₂ and WS₂ samplesmilled for 15 hours in the planetary mill (FIG. 4a ) and the mixer mill(FIG. 4b ).

FIGS. 5a and 5b are HAADF-TEM images of the 1:1 (molar) MoS₂ and WS₂samples milled for 15 hours in the planetary mill (FIG. 5a ) and themixer mill (FIG. 5b ). HAADF-TEM is high-angle annular dark-filedtransmission electron microscopy.

FIG. 6 shows Raman spectra of the 1:1 (molar) MoS₂ and WS₂ samplesmilled for 15 hours in the planetary mill (curve a) and in the mixermill (curve b).

FIG. 7 is the DSC/TGA curve of the 1:1 (molar) MoS₂ and WS₂ samplesmilled for 15 hours in the planetary mill.

FIG. 8 shows powder X-ray diffraction patterns of the 1:1 (molar)mixture of MoS₂ and WS₂ milled for 15 hours in a planetary mill (curvea—as-milled) and then heat treated at 950 degrees C. in argon for 16hours (curve b—annealed).

FIGS. 9a and 9b are HAADF-TEM images of the 1:1 (molar) MoS₂ and WS₂samples milled for 15 hours in the planetary mill and then heat treatedat 950 degrees C. in argon for 16 hours.

FIGS. 10a and 10b are powder X-ray diffraction patterns of the 1:1(molar) mixture of MoS₂ and WSe₂ (FIG. 10a ) and MoSe₂ and WS₂ (FIG. 10b). Both materials were ball milled for 15 hours in a planetary mill andthen heat treated at 1000 degrees C. in argon for 16 hours.

FIG. 11 is Raman spectra of the 1:1 (molar) MoS₂—WSe₂ samples milled for30 hours in the planetary mill and then heat treated at 1000 degrees C.in argon for 16 hours.

FIG. 12 shows powder X-ray diffraction patterns of the starting MoS₂ andWSe₂, and the 1:1 (molar) MoS₂—WSe₂ sample milled for 30 hours in aplanetary mill at 600 rpm.

FIG. 13 is HAADF-TEM and HAADF-EDS images of the 1:1 (molar) MoS₂ andWSe₂ samples milled for 30 hours in the planetary mill.

FIG. 14 is the high-resolution XPS spectra of Mo 3d, S 2p, Se 3d and W4f for the 1:1 (molar) MoS₂ and WSe₂ sample milled for 30 hours in theplanetary mill and then heat treated at 1000 degrees C. in argon for 16hours.

FIG. 15 shows powder X-ray diffraction patterns of the starting MoS₂ andWSe₂, and the 1:1 (molar) MoS₂—WSe₂ samples milled for 1 and 4 hours inthe mixer mill.

FIG. 16 is powder X-ray diffraction patterns of the starting MoS₂ andMoSe₂, and the 1:1 (molar) MoS₂—MoSe₂ samples milled for 30 hours in theplanetary mill and then heat treated at 1000 degrees C. in argon for 16hours.

FIG. 17 is Raman spectra of the starting MoS₂ and MoSe₂, and the 1:1(molar) MoS₂—MoSe₂ samples milled for 30 hours in the planetary mill andthen heat treated at 1000 degrees C. in argon for 16 hours.

FIGS. 18a -18 b; 19 a-19 b, and 20 a-20 b relate to five- andsix-principal element TMDCs, wherein FIG. 18a shows XRD pattern and FIG.18b shows Raman spectra of (Mo_(0.4)W_(0.2)Nb_(0.4))S_(0.8)Se_(1.2).Corresponding data for (Mo_(0.6)W_(0.2)Ta_(0.2))S_(0.8)Se_(1.2) is shownin FIG. 19a and FIG. 19b , and for(Mo_(0.25)W_(0.25)Nb_(0.25)Ta_(0.25))SSe is shown in FIG. 20a and FIG.20 b. Diffraction patterns and Raman spectra of starting materials areshown as references.

FIG. 21 shows powder X-ray diffraction patterns of the 1:1 (molar)MoS₂—WSe₂ samples milled for 1, 10 and 30 hours in the planetary mill at300 rpm.

FIGS. 22a-22e show HAADF-STEM and STEM-EDS images of the as-milledequimolar mixture of MoS₂ and WSe₂; wherein FIG. 22a shows an example ofa “compositionally uniform” particle and FIG. 22b shows aheterostructured particle. A side view of a large particle is shown inFIG. 22c and FIG. 22d , and a “reshuffled” fragment of the same materialis shown in FIG. 22e . Due to the higher Z-factor, WSe₂ fragmentsproduce brighter image than the MoS₂ layer

FIG. 23 is powder X-ray diffraction patterns of the 1:1 (molar)MoS₂—WSe₂ samples milled for 30 hours in the planetary mill at 300 and600 rpm and then heat treated at 1000 degrees C. in argon for 16 hours.

FIG. 24 is a schematic diagram of liquid-phase preparation ofsingle-phase, bulk TMDC materials according to an embodiment of theinvention.

FIG. 25 are powder X-ray diffraction patterns of the 1:1 (molar)MoS₂—WSe₂ sample processed as described in Example 4 before heating.

FIG. 26 are powder X-ray diffraction patterns of the 1:1 (molar)MoS₂—WSe₂ sample processed as described in Example 4 after heating at1000 degrees C. in argon for 16 hours.

FIG. 27 are powder X-ray diffraction patterns of the 1:1 (molar)MoS₂—WSe₂ sample processed as described in Example 5 before heating.

FIG. 28 are powder X-ray diffraction patterns of the 1:1 (molar)MoS₂—WSe₂ sample processed as described in Example 6 before heating.

FIG. 29 are powder X-ray diffraction patterns of the 1:1 (molar)MoS₂—WSe₂ sample processed as described in Example 6 after heating at1000 degrees C. in argon for 16 hours.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides method embodiments for preparing atransition metal chalcogenide material, such as a TMDC, by exfoliatingtwo or more different starting TMDCs separately or together andcombining the exfoliated products to form a transition metalchalcogenide structure having layers of different composition. Thecombining of the exfoliated products can be achieved in a passive mannerby self-assembly in the event the TMDCs are exfoliated together and/orin an active manner by mixing the products together in the event theTMDCs are exfoliated separately.

The present invention involves preparing a transition metal chalcogenidematerial starting for example from different, bulk, starting TMDCswherein the method includes one or more solid-state or liquid-assistedexfoliation steps. For purposes of illustration and not limitation,Examples 1-3 set forth below describe dry mechanical exfoliation bymechanical processing to this end. For purposes of illustration and notlimitation, Examples 4-7 describe liquid-assisted mechanical exfoliationby mechanical processing, which can be followed by liquid phasesonication, to this same end.

In practice of embodiments of the present invention, the bulk TMDCs thatare subjected to processing pursuant to the present invention caninclude the TMDCs selected from the group consisting of the group 4TMDCs (M=Ti, Zr, Hf), group 5 TMDCs (M=V, Nb, Ta) or group 6 TMDCs(M=Cr, Mo, W), group 7 TMDCs (M=Mn, Re), group 10 TMDCs (M=Pd, Pt)],group 11 TMDCs (Cu, Ag), group 12 TMDCs (Zn, Cd), group 13 TMDCs (e.g.,M=In, Ga) as well as lanthanum group metals chalcogenides (Sc, Y, La,Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu).

Practice of embodiments of the invention can produce mixed transitionmetal dichalcogenide materials that include two or more metals selectedfrom the group consisting of metal is selected from the group consistingof Ti, Zr, Hf, Nb, Ta, Mo, W, Re, Pd, Pt, In, Ga and Cr and two or morechalcogen elements, namely, two or more of S, Se, and Te. Bulk layeredchalcogenide materials that can be processed pursuant to the methodembodiments can include, but are not limited to two or more bulksingle-phase commercially available TMDCs as well as optional graphene,boron nitride, black phosphorus and other related layered materials thatadditionally and optionally may be incorporated into the composition ofa final metal multi-chalcogenide composite product by exfoliation withthe TMDCs wherein the composite structure is built-up of 2D exfoliatedlayers of the different materials.

Examples 1-3 describe illustrative embodiments involving solid-statemechanical processing which can be conducted using pestle and mortar,shaker ball mills of any configuration, planetary ball mills of anyconfigurations, any type of laboratory or industrial grinders, or othermilling, or grinding equipment that can produce plastic, shear and otherirreversible deformation as well as partial or complete exfoliation ofthe bulk TMDCs to an extent that achieves the results described below.The mechanical processing can be conducted under an inert ornon-reactive gas atmosphere and at any temperature including below roomtemperature, at room temperature, and/or at elevated temperatures aboveroom temperature such as to 100-1500° C. for appropriate times such asincluding, but not limited to, 1 minute to 72 hours or longer. Themechanical processing can be followed by heat treatment at elevatedtemperatures above room temperature such as 100-1500° C. A TMDC having adisordered, heterogeneous-layered structure or ordered, single-phasestructure can be produced depending upon processing temperature andprocessing time. The ordered, single-phase TMDC can be a substantiallyhomogenous solid solution of two or more different TMDCs.

Examples 4-7 describe illustrative embodiments involving (1)mechanochemical exfoliation of two or more different TMDC materials inthe same or different appropriate liquid environment using anyexfoliation technique, step (2) subsequent generation of mixedhierarchical (layered) heterostructures, either by a spontaneousassembly in solution (e.g. mixing together of the dispersions formed instep (1)) or by using an additive assembly technique (e.g.layer-by-layer deposition, spin coating, ink jet printing etc. of thedispersions formed in step (1), and then step (3) reactive interlayermixing and conversion of the hierarchical (layered) heterostructuresobtained in step (2) into a single phase material, wherein reactiveintermixing is effected by means of heat treatment, or mechanicalworking/processing, or high uniaxial external pressure, or a combinationof external pressure and shear stress, or by cold or hot rolling or bycombination of thereof. The invention envisions preparing a TMDCmaterial using steps (1) to (3). The material so formed can include amixed metal component.

In illustrative embodiments, the exfoliating step is conducted bymechanical processing in the same or different liquid(s) to formdispersion(s) and the combining step involves spontaneous self-assembly(e.g. by mixing the dispersions together) or additive assembly (e.g.layer-by-layer deposition, spin coating, ink jet printing, etc. of thedispersions) of the exfoliated products of the dispersions.

The intermediate 3D heterostructured TMDC material having hetero-layers(heterogeneous layers) can be converted to a single phase by subjectingthe material to further processing including, but not limited to,thermal heat treatment (annealing), milling, grinding, cold or hotrolling, extrusion, high hydrostatic external pressure or combinationthereof to obtain an ordered single-phase material.

Practice of illustrative method embodiments enables simple, inexpensive,and scalable way of industrial production of single-phase materialsdescribed above. The liquid media employed for the liquid-assistedembodiment described above can be selected from the list of inexpensive,mass-produced organic solvents (e.g. isopropanol and other alcohols).

The following Examples are offered to further illustrate embodiments ofthe invention without limiting the scope of method embodiments.

Preparation of Three and Four Principal Element (Mixed Metal) TMDCs bySolid-State Mechanical Exfoliation and Annealing:

Materials and Analytical Techniques.

Ultra-high purity Ar (Matheson, 99.999%), ultra-high purity He(Matheson, 99.999%), MoS₂(Sigma-Alrdich, 99% purity), WS₂(Sigma-Alrdich, 99% purity), MoSe₂ (Alfa Aesar, 99.9% purity), WSe₂(Alfa Aesar, 99.8% purity), and NbSe₂ (Alfa Aesar, 99.8% purity) wereused. As-received commercially available transition metaldichalcogenides, MoS₂, WS₂, MoSe₂, WSe₂ and NbSe₂, were used as startingmaterials without further purification. TaS₂ was prepared from Ta powder(99.99% purity) and S (Alfa Aesar, 99.9995% purity). In particular, TaS₂was prepared by ball milling a nearly stoichiometric mixture of tantalumand sulfur (5% excess). The milling was performed for 4 hours in aFritsch, Pulverisette 7 milling machine, then followed by heat treatmentof the resulting powder in a quartz tube sealed under 0.75 bar pressureof high purity helium for 5 days.

Powder X-ray Diffraction (XRD): The measurements were carried out usinga PANalytical X'PERT powder diffractometer with an Xcelerator detectorin the 2θ range from 10° to 80° with 0.02° step size employing Cu—K_(α1)radiation (λ=0.15406 nm).

Thermogravimetric Analysis (TGA): TGA data were collected using NetzschLuxx STA 409 PG. About 10 mg of the investigated materials were placedin alumina crucibles and heated up under argon from room temperature to1450° C. with a ramping rate of 10° C./min.

HAADF-STEM and STEM-EDS measurements: TEM experiments were performed ona Titan Themis (FEI) probe Cs-corrected TEM. High-resolution HAADF-STEMimages were collected by the convergence semi-angle of 18 mrad and acollection semi-angle of 99-200 mrad at 200 kV. STEM-EDS analysis wascarried out using a Super-X EDS detector attached to the Titan Themis.Focused Ion Beam (FIB) cross-sectioning was performed on a HeliosNanolab G3 UC dual-beam instrument (FEI). A liftout for TEM was carriedout using standard in-situ TEM liftout procedures by means of the EZLiftmicromanipulator and Multi-Chem gas injection system [38, 39]. For theFIB cross-sectioning, a large agglomerate of particles was selected andcovered with a protective layer of carbon that prevented the sputteringof the top surface of the sample during the experiment. Next, a trenchwas sputtered from both sides of the particle, resulting in arectangular shape specimen, which afterwards was attached to a tungstenneedle and thinned to electron transparency.

Raman Spectroscopy: Powdered samples were spread on glass cover slipsand analysed with a Horiba XploRA Raman microscope (HORIBA Scientific,Edison, N.J.) using 532-nm excitation (8.3·10³ W/cm²) and a 100×(0.90NA) objective. The detector was a front-illuminated Horiba Synapse EMCCDcamera, and the acquisition time was 60 seconds. For each sample, thedisplayed spectrum was an average of 10 locations.

High-resolution X-ray photoelectron spectra (XPS): Measurements werecarried out for compositional analysis. A few milligrams of powdersamples were mounted on a double-sided Scotch tape. XPS spectra for S2p, Se 3d, Mo 3d and W 4f were acquired using physical electronic 5500multi technique system with an Al—K_(α) source. To compensate thecharging effects, the binding energies of all peaks were corrected usingstandard carbon peak at 285 eV.

EXAMPLE 1 MoS₂—WS₂ System

In a typical experiment, a total 1 g of the stoichiometric mixture ofMoS₂ powder and WS₂ powder in the 1:1 molar ratio was transferred to amilling container together with eight 12 mm stainless steel balls (about7 g each) so that ball-to-sample ratio was close to 56:1. After sealingthe container under argon, ball milling was carried out for various timeintervals at 600 rpm in a horizontal planetary mill Fritsch,Pulverisette 7. To facilitate the uniform milling and prevent caking ofthe powder during the processing, the milling mode was alternatedbetween forward and reverse (30 min each) with an intermittent pause of5 min. To investigate the effect of milling regime, the same amount ofthe sample was also processed in a high-energy mixer mill (SPEX, 8000Mmill) in a stainless steel container with two 12.7 mm and four 6.35 mmgrinding balls (ball-to-sample ratio=18:1). Ball milled powders werecharacterized using powder X-ray diffraction (XRD), differentialscanning calorimetry and the thermogravimetric analysis (DSC/TGA), Ramanspectroscopy, high-angle annular dark-field scanning transmissionelectron microscopy (HAADF-STEM), scanning transmission electronmicroscopy energy dispersive spectrometry (STEM-EDS), scanning electronmicroscopy (SEM), scanning electron microscopy energy dispersivespectrometry (SEM-EDS) and scanning and transmission electronmicroscopies (SEM and TEM) as appropriate. In each example, milling wasconducted at ambient temperature.

XRD for the samples processed for different periods of time in planetaryand mixer mills are shown in FIGS. 1a and 1 b, respectively. Due toidentical crystal structure and very close cell parameters for MoS₂ andWS₂, no shift in the Bragg peak positions could be observed even after aprolonged milling. However, strong peak broadening that becomes moreapparent with the increasing milling time, which is a typical signatureof disordering in the sampleis clearly observed.

SEM evaluation of two samples milled for 15 hours in the planetary milland the mixer mill are shown in FIG. 2a, 2b . They revealed particleagglomeration on a microscopic scale. The particle morphology is highlyirregular, with a broad particle size distribution. The distribution ofconstituting elements within the particles was analyzed using thebackscattering electron imaging (BEI) combined with EDS. Backscatteringanalysis suggests uniform composition; i.e. no separate particles orregions of high concentrations of MoS₂ or WS₂ were present (FIG. 2c, 2d). Both samples on the micrometer-size scale appeared homogeneous.

Analysis by SEM-EDS showed the characteristic peaks for tungsten,molybdenum, sulfur and carbon. The latter coming from the sample holdergrid. As can be seen in FIG. 3, SEM-EDS confirms an even distribution ofW, Mo, and S throughout the particle(s). Unfortunately, some overlappingpeaks of S K- and Mo L-series in the SEM-EDS spectrum made the analysisqualitative. Therefore, the signal coming from W appears to be strongercompared to Mo. Similar results were obtained for the samples processedin the mixer mill (not shown).

For samples processed either in the planetary or mixer mills, TEM (FIG.4a, 4b ) reveals disordered layered structures, which are consistentwith the broadening of Bragg peaks in their XRD patterns (FIGS. 1 a, 1b). Again, the uniform layer spacing of about 0.6 nm, characteristic forboth MoS₂ and WS₂ (002) basal plane, cannot be indicative of a solidsolution or layered hetero-structures due to the similarity of the MoS₂and WS₂ cell parameters.

The HAADF-STEM studies confirm disordering in both milled samples (FIG.5a, 5b ). The images clearly show high degrees of distortion and poorcrystallinity of the as-milled materials. The Z-dependence visualizesW-rich sites as white to light-grey zones, and Mo-rich—as grey todark-grey zones. Based on the Z-contrast images, in both cases a solidsolution of MoS₂ and WS₂ may be suggested.

However, the Raman spectrum of the sample (FIG. 6) does not agree withthat published for a single-phase (Mo_(1-x)W_(x))S₂ [40] On thecontrary, it contains a set of broad peaks at 352, 380, 407 and 418 cm⁻¹that resemble those of the individual MoS₂ and WS₂ phases in laminarMoS₂/WS₂ heterostructures. [41] Annealing the ball-milled powders at1000° C. for 16 hours under helium produces a fused single-phasematerial, whose Raman spectrum (FIG. 6) matches that of(Mo_(0.5)W_(0.5))S₂.

In order to obtain a crystalline material, the sample milled in aplanetary ball mill for 15 hours was heat treated at 950 degrees C. Theannealing temperature of 950 degrees C. was selected based on theresults of DCS/TGA analysis for pure binary TMDCs (FIG. 7) andanalytical data published in open literature. At ambient pressure, bulkMoS₂, WS₂ and WSe₂ are fairly stable up to ˜1100 degrees C., but theyslowly release chalcogens between 800-1000 degrees C. in high vacuum[42,43]. This suggests that all three binary TMDCs are approaching theedge of their stability at T 950-1000 degrees C., where mobile reactive{MX_(Y)} species formed can drive their chemical conversion.

The powder XRD pattern of the material heat treated for 16 hours in anargon atmosphere consisted of sharp crystalline peaks (FIG. 8).Significantly improved crystallinity of the material was also confirmedby the HAADF-STEM measurements (FIGS. 9a, 9b ). The heat treated sampleconsists of a 2H polymorph. Both HAADF- and EDS-STEM indicated a uniformdistribution of Mo and W throughout the lattice,

EXAMPLE 2 MoSe₂—WS₂ and MoS₂—WSe₂ Systems

Processing of both MoS₂—WSe₂ and MoSe₂—WS₂ systems was performed in aplanetary mill. For this purpose, 2 g of corresponding equimolar mixturewas ball milled at 600 rpm in a planetary mill for 30 hours with eight12 mm stainless steel balls. The XRD traces of the samples obtained(FIGS. 10a, 10b ) showed a significant Bragg peak broadening indicativeof disordering in the starting materials. Partially incompletehomogenization of the as-milled samples can be concluded from theenhanced Bragg scattering corresponding to the individual chalcogenides.The 1:1 (molar) MoS₂—WSe₂ is clearly more homogeneous when compared tothe 1:1 (molar) MoSe₂—WS₂ system.

After the heat treatment at 1000 degrees C. for 16 hours in argon, bothof the investigated materials crystalized into mixed TMDC systems withcharacteristic Bragg peaks (FIG. 10a for MoS₂—WSe₂ and FIG. 10b forMoSe₂—WS₂) that correspond to Mo_(0.5)W_(0.5)S₀₅Se_(0.5). In the case ofMoSe₂—WS₂ system, both the formation of the solid solution and thecrystallization were incomplete as evidenced by the high-angle shoulderof the Bragg peak at 2θ≈15 degrees and overall broader Bragg peaks whencompared to the MoS₂—WSe₂ sample.

The Raman spectrum of the heat treated material confirms the XRD results(FIG. 11). In the case of MoS₂—WSe₂ system, it consists of threecharacteristic broad bands at about 265, 355 and 400 cm⁻¹, clearlydeviating from those observed in the starting MoS₂, WSe₂ and theas-milled sample. The XRD pattern of the latter indicates the presenceof both MoS₂ and WSe₂ phases in the material even after 30 hours of ballmilling (FIG. 12). Once again, STEM-EDS suggests compositionalhomogeneity of heat treated samples (FIG. 13).

The high-resolution X-ray photoelectron spectroscopy (XPS) (FIG. 14)reveals the presence of all four-principal elements, Mo, W, S and Se, onthe surface of the heat treated material. The XPS spectrum of theannealed (Mo_(0.5)W_(0.5))SSe contains peaks at 229.5 and 232.6 eVcorresponding to Mo⁴⁺(3d_(5/2)) and Mo⁴⁺(3d_(3/2)) ions and the signalat 227 eV agrees with the position characteristic for S²⁻(2s) ion [44,45]. The peaks at 32.9 and 35.1 eV are indicative of W⁴⁺(4f_(7/2) and4f_(5/2)) in TMDCs [46] while the weak signal at 38 eV suggests thepresence of a minor W⁶⁺(WO₃) contamination [44]. Finally, the peakaround 162.5 eV corresponds to the unresolved signals byS²⁻(2p_(3/2))/S²⁻(2p_(1/2)). The additional peaks at ˜161 and ˜167 eVbelong to Se²⁻(3p_(3/2)) and Se²⁻(3p_(1/2)), respectively [47]. Thesignal at ˜55 eV can be assigned to overlapping 3d_(5/2) and 3d_(3/2)peaks of Se²⁻.

It is worth noting that the frequency of milling has a distinct effecton its result. Thus, the XRD pattern of the materials processed for 30hours in the planetary mill at 300 rpm (processing frequency of 5 Hz) isquite similar to that obtained after one hour of milling of the MoS₂ andWSe₂ mixture in a SPEX 8000M unit (processing frequency of ˜18 Hz¹), andthe XRD pattern of the sample processed for 4 hours in the SPEX 8000Mmill closely resembles that of the powder generated in the FritschPulverisette 7 planetary mill at 600 rpm (the processing frequency of 10Hz) for 30 hours (FIG. 15).

The material of the milling equipment does not seem to affect theoverall outcome of the synthetic process and a single-phase(Mo_(0.5)W_(0.5))SSe could be successfully prepared in both siliconnitride and hardened steel vials after heat treatment. In the lattercase, processed material contained up to 0.4 at. % of iron contaminationafter 30 hours of milling as determined by the X-ray fluorescencespectrometry (XRF). Similar amount of iron was also discovered in otherTMDC samples prepared using the hardened steel milling sets.

(Mo_(0.5)W_(0.5))SSe crystallizes in space group P6₃/mmc where Mo and Watoms randomly occupy the 2c position (1/3, 2/3, 1/4) in the crystallattice. The 4f site (1/3, 2/3, z) is filled randomly with S and Se.Shapes of Bragg reflections in the XRD pattern are highly anisotropicowing to distinctly plate-shaped particles and a nonrandom distributionof their orientations, which causes texturing and asymmetric broadeningof the (h 0 l) Bragg peaks. Correcting for these effects requiresspherical harmonics expansion to approximate crystallite shapes forRietveld-based refinements. Results of which for this and othercompounds described in this work are listed in Table 1.

TABLE 1 Structural parameters of TMDCs derived from Rietveldrefinements. The space group is P6₃/mmc (#194). Metal atoms (Mo, W, Nbor Ta) occupy 2c (⅓, ⅔, ¼) site and chalcogens occupy 4f (⅓, ⅔, z) site.Standard deviations are given in parentheses. Column labelled R_(p)lists profile residuals. Lattice parameters Phase Composition a, Å c, ÅChalcogen z/c R_(p) Mo_(0.5)W_(0.5)S₂ 3.1628(1) 12.3581(4) 0.6229(2)6.77 Mo_(0.5)W_(0.5)SSe 3.2239(5) 12.7348(3) 0.6169(2) 6.99 MoSSe3.2246(4) 12.7069(2) 0.6168(1) 9.10Mo_(0.4)W_(0.2)Nb_(0.4)S_(0.8)Se_(1.2) 3.3073(2) 12.5718(9) 0.6151(2)8.86 Mo_(0.6)W_(0.2)Ta_(0.2)S_(0.8)Se_(1.2) 3.1754(1) 12.4158(2)0.6186(3) 9.01 Mo_(0.25)W_(0.25)Nb_(0.25)Ta_(0.25)SSe 3.3015(2)12.5189(9) 0.6223(3) 8.82

EXAMPLE 3 Three to Six Principal Element (Mixed Metal) TMDC Systems

In a typical experiment, a 1 or 2 g sample of a physical mixture of twoor more different binary TMDCs, taken in an appropriate stoichiometricproportion, was milled in either stainless steel milling container witheight 11.9 mm stainless steel balls, or in a silicon nitride vial withthree 12.7 mm silicon nitride grinding balls using a two-stationhorizontal planetary mill (Fritsch, Pulverisette 7), or a shaker mill(SPEX 8000M) for various periods of time (1-30 hours). The millingcontainers were loaded and sealed under ultra-high purity argon in aglove box. To facilitate uniform milling and to prevent kinking of thepowder during the processing, the milling mode of the planetary mill wasalternated between forward and reverse rotations (30 min each) with anintermittent pause of 5 min. Subsequently, as-milled powders werepressed into pellets under argon in a glove box, placed in a quartztube, which was further sealed under 0.75 bar of ultra-high purityhelium. Typically, the heat treatment was conducted by ramping thetemperature to 1000° C. and annealing the material for 16 hours orlonger. Afterwards, samples were allowed to cool down to roomtemperature in the furnace. For analytical characterization, preparedmaterials were crushed in a mortar with a pestle and stored in a glovebox under high-purity argon. The material of the milling equipment doesnot affect the overall outcome of the synthetic process. Also, accordingto X-ray fluorescence spectrometry (XRF), samples prepared using thehardened steel setup contained up to 0.4 at. % of iron contamination.

Ball milling of the equimolar mixture of bulk MoS₂ and MoSe₂, followedby heat treatment of the resulting powder, reliably and reproduciblyyields the known three-element chalcogenide, MoSSe. In this case, thepresence of the starting materials in the as-milled powder was confirmedby both XRD and Raman spectroscopy, while a single-phase MoSSe formsafter annealing at 1000° C. (FIG. 16, 17).

Starting from bulk MoS₂, WSe₂ and NbSe₂, a five-element compound withthe nominal composition of (Mo_(0.4)W_(0.2)Nb_(0.4))S_(0.8)Se_(1.2)(FIG. 18a, 18b ) was prepared. Similar to other discussed cases, afterball-milling for 30 h, the material emerges as a highly disorderedmulti-phase powder. Its Raman spectrum (FIG. 18b ) contains twoprominent peaks at 378 and 404 cm⁻¹, matching MoS₂, and the very broadsignal at ˜250 cm⁻¹ that combines characteristic peaks of NbSe₂ and WSe₂at 228, 236 and 248 cm⁻¹. Subsequent annealing at 1000° C. converts theas-milled powder into a fused single-phase(Mo_(0.4)W_(0.2)Nb_(0.4))S_(0.8)Se_(1.2).

Further, (Mo_(0.6)W_(0.2)Ta_(0.2))S_(0.8)Se_(1.2) (FIG. 19a, 19b ) wasprepared from a stoichiometric mixture of MoSe₂, WS₂ and TaS₂ using thesame procedure. However, in this instance, an extended annealing time of72 hours was necessary to obtain a crystalline material suitable forRietveld refinement.

Finally, the six-component compound,(Mo_(0.25)W_(0.25)Nb_(0.25)Ta_(0.25))SSe (FIG. 20 a, 20 b) has beenprepared by ball milling and subsequent annealing of an equimolar(1:1:1:1) mixture of MoS₂, WSe₂, NbSe₂ and TaS₂.

The Bragg peaks in the XRD patterns of all five- and six-principalelement (mixed metal TMDCs remain substantially broadened even after aprolonged annealing, and a minor oxide impurity was detected in bothTa-containing samples. Reasonably assuming similarity of particle sizesand shapes in all three materials prepared from similar precursors, theobserved Bragg peak broadening can be attributed to reducedcrystallinity due to built-in strain. Since both Nb and Ta are largerthan Mo and W, combining them in the same metallic layer of amulti-element TMDC should cause distinct distortion of the layers thatpropagates into the entire 3D-lattice.

In achieving the results described above, the tangential component ofball-milling, which is accountable for the shearing action, appears toenable mechanical exfoliation of bulk TMDCs. At the same time, theexfoliated TMDCs can easily restore their 3D-arrangements by restacking,which is used to construct vertical 3D-heterostructures from exfoliatedtwo dimensional TMDCs, graphene, h-BN and similar single-layernanomaterials [49, 50]. Hence, it is quite feasible that mechanicalexfoliation and spontaneous restacking of different TMDCs in a waysimilar to reshuffling a deck of playing cards, can produce 3Dhetero-assemblies that appear uniform for EDS but, in fact, areheterostructured materials. The latter can further transform intouniform single-phase materials during subsequent heat treatment.However, the inventors do not intend to be bound by any above theory orabove explanation.

Experiments using lower-speed (300 rpm) ball milling of bulk MoS₂ andWSe₂ shed some additional light upon layer-reshuffling duringball-milling. Contrary to the more intense (600 rpm) processing, bothMoS₂ and WSe₂ phases remain clearly distinguishable in the XRD patternsof the as-milled powders even after 30 hours of milling (FIG. 21), andthe STEM-EDS analysis now clearly reveals the presence of two differenttypes of particles in this material (FIG. 22a-22e ). The first kind ofparticles looks compositionally uniform (FIG. 22a ) resembling particlesin the as-milled samples discussed above. The second group consists ofhetero-assemblies, where the “reshuffling” remains incomplete, andindividual MoS₂ and WSe₂ segments can be distinguished around the areasof their overlap (FIG. 22b ). The Focused Ion Beam (FIB)cross-sectioning experiment performed on a large agglomerate of theparticles, which was cut by a Ga-ion beam, attached to a tungsten needleand rotated in the TEM chamber to produce a side view, clearlydemonstrates its heterogeneous layered structure (FIG. 22 c, 22 d).Heating the as-milled, incompletely reshuffled powder at 1000 degrees C.for 16 hours produces a fused single-phase (Mo_(0.5)W_(0.5))SSeidentical to the material obtained in other experiments. (FIG. 23)

Thus, available experimental data strongly suggest that mechanicalmilling of MoS₂ with WSe₂ leads to stochastic rearrangement of bothindividual layers and incompletely exfoliated slabs into 3Dhetero-assemblies where both kinds of building blocks retain theircompositional individuality (FIG. 22d, 22e ).

The above examples demonstrate that the method embodiments of theinvention described above enable easy and reliable preparation ofdiverse multi-principal element (mixed metal) TMDCs that were eitherunknown or barely accessible via conventional materials fabricationroutes.

Mechanochemical exfoliation coupled with stochastic restacking of binaryprecursors enables the generation of layered 3D heterostructures, andannealing of the latter generates single-phase multi-metal elementmaterials. It is quite feasible that similar or closely relatedprotocols can be applied to the preparation of other classes ofmaterials, which are inaccessible or hard-to-reach through conventionalsynthetic routes.

Another important outcome of practice of method embodiments is thatmechanochemical treatment facilitates the formation of3D-heterostructures from bulk TMDCs that may provide a path to prepareto a broad range of unusual hetero-structured nanomaterials.

Finally, multi-principal element (mixed metal) materials synthesizedusing the method embodiments, represent a unique group ofhigh-entropy-like systems that can serve as precursors of new 2Dnanomaterials and 3D-heterostructures with future applications inelectronics, electrochemical water splitting, and advanced lubrication,to name a few.

Preparation of Multi-Principal Element (Mixed Metal) TMDC's byLiquid-Assisted Exfoliation:

FIG. 24 illustrates schematically liquid-phase preparation ofsingle-phase, bulk TMDC materials according to other embodiments of theinvention. After synthesis, the liquid medium can be removed by vacuumevaporation, boiling away, dry freezing or any other technique.

EXAMPLE 4 MoS₂—WSe₂ System

Commercial MoS₂ powder (Sigma-Alrdich, 99% purity) and WSe₂ powder (AlfaAesar, 99.8% purity) were used as starting materials. MoS₂ and WSe₂, 2 gof each powder, were separately ball milled in the presence of 1 ml ofisopropanol for 30 hours using a Fritsch Pulverisette 7 planetary mill.The milling was carried out in separate milling vials sealed under argonwith eight 12 mm stainless steel balls (about 7 g each) at 600 rpm. Themilling mode was alternated between forward and reverse (30 min each)with an intermittent pause of 5 min to facilitate uniform shear milling.

Obtained powders were dried under vacuum and corresponding amounts ofthe samples were weighed out to obtain stoichiometric molar ratioof1MoS₂:1WSe₂ with a total mass of 0.5 gram. Each sample was separatelyadded to 40 grams of isopropanol and sonicated for 60 minutes in a FS20HFisher Scientific sonicating machine. Thereafter, obtained suspensionswere centrifuged at 3500 rpm for 15 min. Equal amounts of the obtaineddispersions were mixed together, then the solvent was evaporated at 70°C., which led to the formation of a mixed solid assembly from exfoliatedMoS₂ and WSe₂ phases as confirmed by XRD (FIG. 25).

Next, the obtained mixed solid material was annealed at 1000 degrees C.in a quartz tube under inert gas atmosphere for 16 hours. The XRDpattern of the heat treated sample showed a highly crystallinesingle-phase solid combining four elements in its structure with thegeneral chemical formula of Mo_(1-x)W_(x)S_(2-2x)Se_(2x) (FIG. 26)

EXAMPLE 5 MoS₂—WSe₂ System

Starting MoS₂ and WSe₂ were prepared as described in Example 4.Subsequently, they were combined together in an equimolar ratio toobtain a 0.5 gram sample. The sample was suspended in 100 ml ofisopropanol and sonicated for 10 hours in a FS20H Fisher Scientificsonicating machine. Next, 50 ml of isopropanol was added to thesonicated suspension, and it was centrifuged for 30 minutes at 3500 rpm.The liquid phase was separated and the precipitated material was driedin vacuum. The obtained dry solid material was annealed at 1000° C. for16 hours in a quartz tube sealed under inert argon. FIG. 27 shows theXRD pattern of the material before annealing, indicating the presence ofMoS₂ and WSe₂ phases in the sample. After annealing, an XRD patternshowed a highly crystalline solid solution of four component TMDC withthe general chemical composition of Mo_(1-x)W_(x)S_(2-2x)Se_(2x).

EXAMPLE 6 MoS₂—WSe₂ System

A total 2 grams of the equimolar mixture of MoS₂ powder and WSe₂ powderwas shear milled for 30 hours in 1 ml of isopropanol to facilitate theirliquid-assisted exfoliation and restacking. The XRD pattern of theas-milled sample shown in FIG. 28 suggests that the product formedcontains chemically unchanged but clearly disordered precursor phases.The absence of the characteristic peaks of (Mo_(0.5)W_(0.5))SSe phaseimplies that only exfoliation and restacking processes may haveoccurred. After heat treatment at 1000° C. for 16 hours under argon, thematerial has transformed into a crystalline single-phase(Mo_(0.5)W_(0.5))SSe as confirmed by XRD (FIG. 29).

In practicing these and other embodiments of the present invention, heattreating of the disordered layered mixed TMDCs can be conducted atelevated temperatures between 100 and 1500 degrees C. in an inert ornon-reactive gas atmosphere to produce the ordered layered mixed TMDC.For purposes of illustration and not limitation, the heat treating timecan be between 1 minute to 72 hours or longer. Heat treating can beconducted in an inert atmosphere comprise helium, argon, krypton, xenon,nitrogen, methane and any other gas, which shows no reactivity towardsthe binary TMDCs and the produced disordered layered mixed TMDC.

The present invention further envisions a subsequent method step thatinvolves exfoliating the ordered layered mixed TMDC (crystallinematerial) to produce a single layer or multi-layer crystallinenanostructure. Exfoliation can be conducted using sonication in a liquidor any other appropriate exfoliation technique.

Although the present invention has been described with respect tocertain illustrative embodiments and examples for purposes ofillustration and not limitation, those skilled in the art willunderstand that changes and modifications can be made therein within thescope of the present invention as set forth in the appended claims.

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1.-24. (canceled)
 25. A transition metal chalcogenide material having ahetero-layered structure having layers of different composition derivedfrom different transition metal chalcogenides.
 26. The material of claim25 that has been converted to comprise a substantially homogeneous solidsolution of two or more different transition metal chalcogenides.27.-46. (canceled)
 47. The material of claim 25 wherein respectivelayers have at least one of a different transition metal constituent anda chalcogen constituent from the next adjacent layer.
 48. The materialof claim 25 where the hetero-layered structure comprises a mixedtransition metal dichalcogenide material.
 49. The material of claim 25wherein the hetero-layered structure comprises a mixed transition metaltrichalcogenide material.
 50. The material of claim 25 wherein thehetero-layered structure comprise a same transition metal dichalcogenidematerial.
 51. The material of claim 25 wherein the hetero-layeredstructure comprises a same transition metal trichalcogenide material.52. The material of claim 25 wherein the transition metal is selectedfrom the group consisting of Ti, Zr, Hf, Nb, Ta, Mo, W, Re, Pd, Pt, In,Ga and Sn.
 53. The material of claim 25 which includes at least one ofgraphite, black phosphorus, and boron nitride.
 54. The material of claim26 which has been heated to convert the structure to a single-phasestructure.
 55. The material of claim 25 wherein the layers areself-combined, exfoliated layers.
 56. The material of claim 52 having achemical composition represented by (M_(a)M² _(b)M³_(c . . . n))(X_(d)X² _(e)X³ _(f)), where the formula unit includes twoor more different transition metals (M), and X, X² and X³ represent S,Se, or Te, whereby the sum of a+b+c+ . . . n is between 1 and 3 and thesum of d+e+f is between 1 and 6.